Vitrified bonded grinding stone

ABSTRACT

The vitrified bonded grinding stone includes a vitrified bond in which super abrasives such as cubic crystal boron nitride grains or diamond grains and an aggregate are contained. The aggregate is formed by porous ceramics and a substance which closes pores of the porous ceramics and the substance is weaker in strength than the porous ceramics and has a physical property that the substance does not dissolve under a burning temperature of the vitrified bond.

INCORPORATION BY REFERENCE

This application is based on and claims priority under 35 U.S.C. 119with respect to Japanese Application No. 2012-157172 filed on Jul. 13,2012, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a vitrified bonded grinding stone which has alow grinding resistance by mixing aggregate which has strength lowerthan the abrasive grain with the grinding stone.

2. Description of Related Art

As an example of the related arts, a grinding stone which has a largeabrasive grain distance between the grains by mixing aggregate which hasstrength lower than the abrasive grain with the binder in order todecrease the grinding resistance (in other words, reduced the degree ofconcentration of the abrasive grains) has been known. As an example ofthe aggregate disclosed in the Patent Document 1, porous ceramics areused as the aggregate which is easy to be fractured and difficult to beabraded with a ground material or material to be ground.

DOCUMENT LIST OF RELATED ART Patent Document

Patent Document 1: JP2009-83036 A

DISCLOSURE OF INVENTION

However, according to the above grinding stone disclosed in the PatentDocument 1, at the time of burning the binder, a melted binder may beinvaded into the pores of the porous ceramics depending on the type ofthe binder which binds the abrasive grains and the aggregate. This maylead to the hardening of the binder invaded into the pores of the porousceramics due to cooling and further, the porous ceramics are becomingdifficult to be fractured and abrasion with the material to be groundoccurs which leads to the burning due to the grinding and may result inthe interference of originally aimed reducing of the grindingresistance.

This invention was made considering the above issues of the related artand it is an object of the invention to provide a vitrified bondedgrinding stone which can prevent the vitrified bond from the invasioninto the pores of the porous ceramics and further can advance thefracturing of the porous ceramics by closing the pores of the porousceramics by a substance which is weaker than the porous ceramics and yetwhich does not dissolve even under the burning temperature for burningthe vitrified bond.

Means to Solve the Problems

The vitrified bonded grinding stone associated with the inventionaccording to claim 1 is characterized in that a vitrified bondedgrinding stone includes a vitrified bond in which super abrasives suchas cubic crystal boron nitride grains or diamond grains and an aggregateare contained and the aggregate is formed by porous ceramics, and asubstance which closes pores of the porous ceramics and wherein thestrength of the substance is weaker than that of the porous ceramics andthe substance has a physical property that the substance does notdissolve under the burning temperature of the vitrified bond.

Further, according to the invention associated with claim 2, in claim 1,the substance is filled in the pores of the porous ceramics to close thepores thereof.

Effects of the Invention

According to the invention associated with claim 1, since the aggregateis mixed into the vitrified bonded grinding stone, a greater graindistance between the grains of the super abrasives can be obtainedthereby to reduce the grinding resistance. Thus, a longer duration oflife of the grinding stone can be expected. Further, according to theinvention, the aggregate of the grinding stone is formed by porousceramics which are easily fractured when brought into contact with thematerial to be ground and the substance which is formed by a material,strength of which is weaker than the porous ceramics for closing thepores of the porous ceramics. Accordingly, a heat generated uponabrasion between the aggregate and the material to be ground can besuppressed and grinding burning of the material to be ground can beprevented. At the same time, even the vitrified bond is melted duringthe manufacturing of the grinding stone, such molten vitrified bondwould not enter into the pores of the porous ceramics and the fracturingproperty of the porous ceramics cannot be influenced thereby.

According to the invention associated with claim 2, the pores of theporous ceramics are closed by the substance by filling the pores of theceramics with the substance. Thus, the invasion of the vitrified bondinto the pores of the porous ceramics can be easily prevented.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

Various aspects of this invention will become more apparent to thoseskilled in the art from the following detailed description of thepreferred embodiments, when read in light of the accompanying drawings,in which:

FIG. 1 is an overall view of the vitrified bond grinding stone accordingto an embodiment of the invention;

FIG. 2 is an enlarged view of the grinding stone layer of the vitrifiedbonded grinding stone; and

FIG. 3 is an enlarged view of grinding stone layer of the grinding stoneupon grinding process.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The embodiments of the invention will be explained hereinafter withreference to the attached drawings. FIG. 1 shows an overall view of thevitrified bonded grinding stone and FIG. 2 is an enlarged view of thestructure of the vitrified bonded grinding stone at grinding surfacearea thereof.

As shown in FIG. 1, the vitrified bonded grinding stone 10 is formed bya disc shaped core 21 and a ring shaped grinding stone layer 22 fixed toan outer peripheral surface of the core 21 by bonding agent or bysintering. The core 21 is formed by a metal material, such as forexample, copper, aluminum or titan, FRP (fiber reinforced plastic)material or a ceramics (normal grinding stone). The grinding stone layer22 is formed by fixing the ring shaped burnt grinding stone layer ontothe outer peripheral surface of the core 21. Alternately, the grindingstone layer 22 may be formed by adhering a plurality of grinding stonesegments to the outer peripheral surface of the core 21 arranged to be aring shaped profile.

A central bore 23 is provided at the center of the core 21 and isengaged with a boss for centering projecting to an axial end of thegrinding stone shaft of a grinding stone base (not shown). A pluralityof bolt bores 24 is provided around the central bore 23 and thecorresponding number of bolts is inserted onto the bolt bores 24. Thebolts are threaded into screwed bores which are open to the axial end ofthe grinding stone shaft. The vitrified bonded grinding stone 10 isfixed to the grinding stone shaft by inserting the bolts into the boltbores 24 and threading the bolts with the screwed bores.

The grinding stone base and table are provided on a grinding machine(not shown) and are slidably guided in a mutually perpendicularlyintersecting with each other. The vitrified bonded grinding stone 10 isto be installed into the grinding machine. The grinding stone shaft isrotatably mounted on the grinding stone base in an axial line which isin parallel with a longitudinal direction of a material to be ground(work W) which is to be ground by the vitrified bonded grinding stone10. The grinding stone shaft is rotatably driven by a motor provided onthe grinding stone base. A main shaft base and a tail stock are alsoprovided on the grinding stone base and between the main shaft base andthe tail stock, the material to be ground (work W) is rotatablysupported in an axial line which is in parallel with the movingdirection of the table.

As shown in FIG. 2, the grinding stone layer 22 is formed by a superabrasives 12 composed by CBN (cubic boron nitride) grains, an aggregate14 the strength of which is weaker than that of the super abrasives 12and a vitrified bon 16 which connects the super abrasives 12 and theaggregate 14. The aggregate 14 is formed by a base material of porousceramics 14 a and a substance 14 b which closes the openings of thepores of the porous ceramics 14 a to prevent invasion of the vitrifiedbond 16 into the pores of the porous ceramics 14 a. as the substance 14b which closes the pores of the porous ceramics 14 a, a substance, suchas for example, a glassy carbon, which is weaker than the porousceramics 14 a and which does not dissolve at the burning temperature forburning the vitrified bond 16 is used.

The aggregate 14 can be easily obtained by filling the substance 14 b inthe pores of the porous ceramics 14 a under the vacuum atmosphere aftermanufacturing the porous ceramics. Alternatively, upon manufacturing theaggregate 14, the porous ceramics 14 a is formed by burning by mixingthe porous ceramics 14 a with the substance 14 b to close the opening ofthe pores of the porous ceramics 14 a.

The vitrified bond 16 connects the neighboring super abrasives 12,neighboring aggregates 14 and neighboring super abrasives 12 andaggregates 14 by bridging therebetween. Thus an air hole 18 is formedbetween the bridging portions 20. The porosity of the porous ceramics 14a is for example usually between 10% and 80% and preferably between 30%and 60% and in this range fracturing upon grinding can be effectivelygenerated and maintain the strength which can maintain the structure ofthe grinding stone. The average grain diameter of the super abrasives 12formed by CBN is for example, 115 μm (#170) and the average graindiameter of the porous ceramics 14 a is, for example, 100 μm (#200). Inthis case, the average grain diameter of the porous ceramics is about87% of the average grain diameter of the super abrasives 12. Thus, it isexperimentally confirmed that the structural strength of the grindingstone as the aggregate can be maintained by setting the grain diameterof the porous ceramics as the aggregate relative to the grain diameterof the super abrasives 12 to be in the range between 70% and 150%. It isconsidered that the porous ceramics 14 a does not weaken the bridgingportions 20 formed by the vitrified bond 16. Instead of using CBS, adiamond grinding grains can be used.

Next, the manufacturing method of the vitrified bonded grinding stone 10according to the embodiment of the invention will be explainedhereinafter. First, the grinding stone layer 22 is formed by the CBNgrains. In this case, the super abrasives 12 of the CBN, the porousceramics 14 a filled with the substance 14 b such as glassy carbon(which is weaker in strength than the porous ceramics 14 a and whichdoes not dissolve under the burning temperature of the vitrified bond16) and the vitrified bond 16 are mixed with a predetermined proportionwhich is defined in advance and kneaded.

For example, the amount of the porous ceramics 14 a to be used is equalto or less than 50% of the entire volume of the grinding stone layer. Ifthe amount of vitrified bond 16 in volume relative to the amounts Of thesuper abrasives 12 of CBN and the porous ceramics 14 a is excessivelyused, the bridging of the binder (bridging portion 20) between theneighboring porous ceramics 14 a, between the porous ceramics 14 a andneighboring super abrasives 12 is difficult to be formed and on theother hand, if the amount of porous ceramics 14 a relative to thevitrified bond 16 in volume is too small, the concentration degree ofthe super abrasives 12 increases, and accordingly the grindingresistance become big. Therefore, the proportion among the composites isdefined in advance. Thus mixed mixture is filled in the formwork formingthe space corresponding to the ring shaped grinding stone layer 22 andformed by press.

The ring shaped grinding stone layer 22 which has been press-formed isthen drawn from the formwork and burnt under the appropriate burningtemperature of the vitrified bond 16 to manufacture the ring shapedgrinding stone layer 22. At this time, the vitrified bond 16 forms thebridging portions 20 and the air holes 18 between the neighboring superabrasives 12 due to the melting during the burning process. However,since the pores of the porous ceramics 14 a are filled with thesubstance 14 b such as glassy carbon which does not dissolve under theburning temperature of the vitrified bond 16, the invasion of the moltenvitrified bond 16 into the pores of the porous ceramics 14 a can beprevented. Thereafter, thus burnt formed grinding stone layer 22 isfixed to the outer peripheral surface of the core 21 by bonding agent tocomplete the vitrified bonded grinding stone 10.

Thus manufactured vitrified bonded grinding stone 10 has the graindiameter of the porous ceramics 14 a as the aggregate is approximatelyequal to the grain diameter of the super abrasives 12 and accordingly,the porous ceramics 14 a becomes the core of the mesh which thevitrified bond 16 forms, and effectively forms the bridging portions 20between the neighboring porous ceramics 14 a or the neighboring superabrasives and the porous ceramics 14 a. This can strengthen thestructure of the vitrified bonded grinding stone 10 and at the same timeprevent the super abrasives 12 from dropping off by abrasion andelongates the duration of life.

Next, the operation at the grinding machining using the vitrified bondedgrinding stone 10 according to the embodiment of the invention will beexplained hereinafter. First the thus manufactured vitrified bondedgrinding stone 10 is fixed to the grinding stone shaft of the grindingstone base then the grinding stone shaft is driven by the motor torotate the vitrified bonded grinding stone 10. Then the work W to beground supported between the main shaft base and the tail stock (notshown) is rotated in an axial line direction by rotating the main shaftof the main shaft base. Thus, the grinding stone base is advanced in adirection perpendicular to the axial line of the work W to grind thework W.

The position of the aggregate 14 and the position of the outerperipheral surface of the grinding stone (projecting position of thesuper abrasives 12) of the grinding stone layer 22 of the vitrifiedbonded grinding stone 10 are approximately the same as shown in FIG. 2,before the grinding process begins. However, since the aggregate 14 isformed by a porous ceramics 14 a which material is weak and is formedwith many pores, the aggregate 14 is fractured by the contact with thesurface of the work W when grinding operation and as shown in FIG. 3,and retreats from the front end position of the super abrasives 12 whichoppose to the work S and serve as the cutting blade. It is noted thatthe glassy carbon substance 14 b is filled in the pores of the porousceramics 14 a. however, since the substance 14 b is weaker than theceramics 14 a in strength and accordingly, the fracturing property ofthe porous ceramics 14 a will not be influenced by the grindingoperation.

According to the embodiment, since the aggregate 14 is mixed into thegrinding stone layer 22 of the vitrified bonded grinding stone 10, thedistance between the grains of the super abrasives 12 can be widened toreduce the grinding resistance. Further, since the aggregate 14 isstructured by the porous ceramics 14 a which is easy to be fractured bythe contact with the material (work W) to be ground, the porous ceramics14 a is fractured when the ceramics 14 a is brought into contact withthe work W upon grinding and further the fractured ceramics retreatsfrom the grain position, which is the cutting blade position, to preventheat generated when the aggregate 14 and the work W are abraded eachother. This can further prevents grinding bum. Still further, thegrinding efficiency can be improved by forming a chip pocket by thefractured ceramics, in which cutting waste is kept and wasted, and byaccelerating circulation of cooling liquid.

Further, since the substance 14 b which strength is weaker than theceramics 14 a is filled in the pores of the porous ceramics 14 a as theaggregate 14, the melted vitrified bond 16 melted upon manufacturing thegrinding stone would not invade into the pores the porous ceramics 14 a.Accordingly, the fracturing performance of the porous ceramics 14 acannot be impaired by the vitrified bond 16 which normally invaded intothe pores of the porous ceramics 14 a and hardens therein after burning.

According to the embodiment of the invention, as the substance, which isto be filled in the pores of the porous ceramics 14 a, glassy carbon isexampled. However, any substance may be used as long as the strength ofthe substance is weaker than the porous ceramics 14 a and the substancedoes not dissolve under the burning temperature of the vitrified bond16.

The invention is explained with the above embodiments, however, theinvention is not limited to the embodiments explained above, but variousother embodiments or modifications are within the scope of the inventionas far as such are not deviated from the subject matter described in theclaims.

1. A vitrified bonded grinding stone comprising: a vitrified bond inwhich super abrasives such as cubic crystal boron nitride grains ordiamond grains and an aggregate are contained, wherein the aggregate isformed by porous ceramics and a substance which closes pores of theporous ceramics; the substance is weaker in strength than the porousceramics and has a physical property that the substance does notdissolve under a burning temperature of the vitrified bond.
 2. Thevitrified bonded grinding stone according to claim 1, wherein thesubstance is filled in the pores of the porous ceramics to close thepores thereof.